1. Field of the Invention
The invention relates to an apparatus and a method for detecting the weak intensity and frequency of terahertz waves.
2. Description of the Related Art
In this context, the term “terahertz waves” means electromagnetic waves whose frequency is in the range of 1 to 10 THz (where, 1 THz=1012 Hz), i.e., whose wavelength is in the range from 0.03 mm to 0.3 mm submillimeter-waves to the far-infrared region.
The terahertz waves are expected to be applied in a wide range of fields, ranging from basic science fields such as radio astronomy, materials science and bimolecular spectroscopy to practical fields such as security, information communication, the environment and medical care.
Nonetheless, in order to apply the terahertz waves to such a wide range of fields, since the terahertz waves are electromagnetic waves whose the frequency bandwidth is interposed between light (i.e., 1013 to 1015 Hz in frequency) such as infrared rays, visible rays, ultraviolet rays and a radio waves (i.e., 1013 to 1012 Hz in frequency), it is difficult to apply as it is to existing techniques such as optics and electronics.
Various detectors for detecting terahertz waves have been already proposed. In particular, Nonpatent Documents 1 and 2 disclose a detector capable of detecting very weak terahertz waves having the intensity of a few fW (10−15 W). In addition, Patent Document 1 discloses a detector capable of detecting the frequency of terahertz waves.
In the Nonpatent document 1, the detector detects the terahertz waves by using carbon nanotubes on a silicon substrate on which a silicon oxide film is formed. Further, the Nonpatent document 2 is concerned with a terahertz wave detector using superconductivity.
The Patent Document 1 has an object to obtain a spectrum having a good frequency resolution in terms of improved signal-to-noise (S/N) ratio measurements. To achieve such an object, according to the Patent Document 1, as shown in FIG. 1, a main body 51 of the detector includes a substrate 53, and a detecting element portion (i.e., a gap g between metal layers 55 and 56), used as an optical switch device, formed on +Z surface of the substrate 53. The element 60, which is almost equal to the substrate 53 in a refractive index, is provided on the −Z side of the substrate 53 so as not to form a surface reflecting the terahertz pulse waves between the −Z surface of the element 60 and the +Z surface of the substrate 53. The shape of the −Z surface of the element 60 and the thickness of the element 60 are set up such that the terahertz waves incident from a certain region of the −Z surface of the element 60 and then focused in the vicinity of the region of the gap g (i.e., the effective region) does not substantially enter the gap g after the light reflected at the +Z surface of the substrate 53 is reflected at the −Z surface of the element 60 for the first time, or otherwise enters the gap g only after it is additionally reflected two or more times.
[Nonpatent Document 1]    T. Fuse, et. al, “Coulomb peak shifts under terahertz-wave irradiation in carbon nanotube single-electron transistors” Applied Physics Letters 90,013119(2007).
[Nonpatent Document 2]    C. Otani, et. al, “Direct and Indirect Detection of Terahertz Waves using a Nb-based Superconducting Tunnel Junction” Journal of Physics: Conference Series, vol. 43, pp. 1303-1306(2006).
[Patent Document 1]    Japanese Unexamined Patent Application Publication No. 2003-232730“terahertz wave detector”
The terahertz wave detector of the Nonpatent document 1 uses the terahertz response of electrons captured in the impurity level of a silicon oxide film. Therefore, in manufacturing the detector, it is difficult to dispose the carbon nanotube at a desired position with respect to the impurity. Moreover, since the impurity level does not have sharp wavelength selectivity, it is difficult to measure the frequency of the terahertz waves.
According to the Nonpatent document 2, since the terahertz wave detector needs ultralow temperatures of 0.3 to 0.4 K to obtain a high sensitivity, it requires using expensive and large scale helium 3 cryostat.
Further, according to the Patent Document 1, since in the terahertz wave detector, the terahertz waves are absorbed by the element 60, it is difficult to detect a very weak terahertz waves having the intensity of a few fW (10−15 W)